Drives of the above type are known, which substantially comprise an input shaft; and a drive pulley connectable to the input shaft and comprising two half-pulleys defining a V groove of variable size to adjust the winding diameter of a V belt. One of the half-pulleys is movable axially with respect to the other by a mechanical centrifugal control device, or by an electronically controlled actuator, to vary the size of the groove.
Drives of the above type also comprise a driven pulley connected to the drive pulley by the V belt, and which is normally reactive, i.e. defined by two half-pulleys loaded axially towards each other by a spring to automatically adapt the work diameter in inverse manner to that of the drive pulley.
In one known solution, in addition to the spring, the two half-pulleys of the driven pulley are connected to each other by a torque-sensitive axial thrust compensating device for producing an additional axial load component when the drive accelerates. The compensating device is defined by a cam guide integral with one of the half-pulleys, and by a cam follower running inside the guide and integral with the other half-pulley; and the guide is defined by a slot sloping with respect to the axis of the half-pulleys so as to generate an axial load component on the cam follower in response to a drive torque acting on the pulley, and so increase the compression exerted by the half-pulleys on the sides of the belt.
High axial thrust at start-up can thus be achieved, while maintaining the load on the spring, and therefore the axial thrust generated by the spring in low-torque conditions, relatively low, thus improving the efficiency of the drive and reducing wear of the belt.
In the presence of a braking torque, however, known axial thrust compensating devices perform the opposite way, i.e. generate axial thrust in opposition to the load of the spring, thus impairing velocity ratio variation response when decelerating, and the belt is released improperly, thus reducing the efficiency with which the braking torque of the engine is transmitted when decelerating or braking.